1. The Field of the Invention
The present invention relates to light matrix display systems. More particularly, the present invention relates to light display systems wherein a plurality of individual light elements are arranged in an X-Y matrix and each light element is controllably switched on or off so that images such as alphanumeric characters, graphic elements, or pictures are formed on the display.
2. The Background Art
The number of light matrix display systems used throughout the world is increasing every year. Light matrix display systems range in size from extremely large displays used in sports stadiums to show scores, times, and animated color pictures to small systems located on store fronts which display messages such as the time and the temperature in numbers only a few inches high.
Regardless of the size of the display, all presently available light matrix displays use a plurality of light elements arranged in an X-Y matrix. The individual light elements are selectively switched on or off in order to form the image. The individual light element is generally a single light generating device, such as a common incandescent light bulb.
In light matrix display systems it is common to refer to the smallest nondivisible portion of the image-forming area of the display as picture elements or "pixels." In small light matrix display systems, the light element and the pixel may be equivalent. That is, each pixel contains only one element, i.e., one lamp. In more complicated light matrix display systems, such as those providing color images, several light elements, for example, red, blue, and green lamps, may be included in each pixel so that each pixel may appear red, blue, and/or green.
In most light matrix display systems intended to be viewed by large numbers of people, it is generally the practice to use incandescent lamps as the light element. Incandescent lamps are resistive devices which inherently require a relatively large amount of current to produce a suitable light output. Furthermore, incandescent lamps are also susceptible to both electrical and mechanical failure due to the fragile nature of the lamp filament while illuminated.
In previously available light matrix display systems it is common to fabricate a large matrix by using smaller segments, for example a segment containing an 8.times.8 lamp matrix having a total of 64 lamps. Each segment, or bank as it is sometimes referred to, is controlled by a discrete collection of lamp driver circuits housed in a package referred to as a driver pack. Alternatively, another approach commonly used is to mount individual lamp driver circuits, one for each light element, on printed circuit cards which are removably mounted in a rack.
Safety concerns and the restrictions of the National Electrical Code limit the number of lamps which may be driven from one driver pack or driver card rack. Regardless of whether the "driver pack" or "driver rack" approach is adopted, the driver circuits which control the individual light elements must be readily replaceable for reasons which will be clearly explained shortly.
In previously available light matrix display systems the lamp drivers, whether located in a driver pack or on a plurality of driver cards, are usually located some distance away from the lamp segment which is being driven. This distance may vary from a few feet to tens of feet depending upon the type of display. For example, in an 8.times.96 light matrix display (a long narrow display), the display may consist of twelve 8.times.8 segments, where each segment is provided with its own driver pack. The driver packs, as commonly used, are located near a load center and are connected to high current capacity AC supply lines.
Before the widespread use of semiconductor power switching devices, mechanical relays or electro-mechanical machines were used as switching devices. The use of mechanical relays, or electro-mechanical machines resulted in extremely bulky circuits for a light matrix display as well as a circuit which required constant maintenance and was prone to failure. While the use of semiconductor switching devices as control devices has increased the reliably of, and reduced the maintenance required by, light matrix display systems, the use of semiconductor switching devices in the systems available in the prior art has several inherent drawbacks.
Among the many semiconductor switching devices now available, the triac is commonly used in light matrix display systems. As will be appreciated by those skilled in the art, a triac is a device which can control a relatively large current e.g., from a few amps to hundreds of amps, while a current on the order of a few milliamps is applied to the control gate of the device. Furthermore, when provided with proper heat dissipation means, very small triacs may "control" large currents.
In previously available light matrix display systems, on triac would be provided in the driver pack or rack mounted board for each lamp to be driven. As is commonly known, triacs inherently possessed some internal resistance while in their "turned on" state and thus each triac generates some heat. When a plurality of operating triacs are housed within a driver pack which is enclosed to protect it from damage and the environment, the accumulated heat can often cause premature triac failure.
Furthermore, when a triac is located more than a few inches away from its corresponding lamp, such as when a plurality of triacs are grouped in a driver pack or driver rack, it necessitates that heavy gauge conductors be used to connect each individual lamp to its corresponding triac to reduce power losses and avoid heat generation in the wires. The use of heavy gauge conductors running between each lamp and the driver pack makes assembly and maintenance of the light matrix display system awkward and cumbersome.
Moreover, in previously available systems each wire had to be individually attached to each lamp socket by use of a wire nut or similar device. Still further, the switching of relatively high currents over the relatively long conductor runs between the driver pack or card rack and the lamp often causes undesirable inductive surges which both interfere with the signals on nearby wiring (causing flickering of extinguished lamps) and cause failure of the triac itself as well as the lamp.
Even during the normal operation of light matrix display systems, triacs will often fail. Furthermore, it is often the case that lamps (which are expected to regularly fail) will fail because the filament leads short together, thus causing a dramatic momentary high current surge which will destroy the triac. Still further, transient current surges also occur due to fluctuations in line voltages and varying operational conditions. It is also common to find that triacs have failed due to shorting together of the heavy gauge conductors which are "bundled" in large groups running between the driver pack or card rack and the lamps. Overall, triacs are susceptible to failure due to occasional high current surges and due to the cumulative effects of many smaller current surges, the adverse effects of which are multiplied several fold when the triac is operated at a high ambient temperature.
Incandescent lamps generally have an expected life of less than 10,000 hours and thus must be regularly replaced, especially in those systems which operate continuously. In most systems, sockets are used to removably secure the lamps in place and to provide proper electrical contact between the heavy gauge conductors and the lamps. The sockets themselves, however, often fail due to physical damage, wear, or exposure to a harsh environment.
Still further, in previously available systems, the driver packs, or driver cards, have both high current devices (triacs) and low current control circuitry (such as digital devices) located side by side. Positioning the low voltage control circuitry in both electrical and physical proximity to the high current devices increases the likelihood of a failure in the low voltage control circuitry.
In the previously available light matrix display systems the usual troubleshooting procedure is to test the lamp, the socket, and the triac associated with the non-operative lamp, in that order. If neither the lamp or the socket is at fault, it is necessary to replace the entire driver pack, or in the case of a "rack driver" the entire driver card associated with the non-operative lamp, while the suspect unit is taken back to the shop and subjected to further troubleshooting procedures.
Following this procedure, the technician must often check the lamp and socket location and then move to the driver pack/card rack location to complete the check of the circuit. This procedure is carred out for each inoperative lamp. Thus, a technician performing maintenance on a light matrix display system must expend a substantial amount of time moving between the lamps on the segment and the driver pack, and he or she must carry a substantial number of driver packs, or driver cards, to use as replacements. A repair technician is required to carry a valuable inventory of complete replacement drivers even though it is most likely that only a single triac within the driver unit requires replacement.